The CETUS Probe Mission Concept 1.5m Optical Telescope Assembly: A high A-Omega approach for ultraviolet astrophysics

We describe the 1.5-m Cosmic Evolution Through Ultraviolet Spectroscopy (CETUS) Optical Telescope Assembly (OTA), a Three Mirror Anastigmat (TMA), providing a large usable focus, which permits non-shared locations for several Ultraviolet (UV) instruments. NASA has selected CETUS as a Probe Mission Concept for consideration by The Decadal Survey ASTRO2020. CETUS will fly in a L2 halo orbit and typically be pointing between 85 degrees and 135 degrees from the sun, and looking at galaxies at redshifts between z=1 and z=2. However, the CETUS payload also will be able to rapidly slew to sun angles between 85 degrees and 180 degrees to reach objects of opportunity, an example of which is a neutron star merger event. CETUS thermal stability starts with lightweighted ZERODUR® mirrors, that are an excellent thermal match to a metering structure of carbon fiber reinforced polymer (CFRP) M55J. This basic passive athermalization approach will be supplemented with controlled heaters, especially at metallic mounts, composite terminations and mechanisms. After launch, solid body metering errors will be optimized by an actuated hexapod in the secondary mirror assembly (SMA). Thus the CETUS telescope can respond to any pointing induced change in solar view factors. Contamination is managed by commissioning heaters radiating to each mirror surface, and a capping shutter over the telescope aperture. The instruments include a wide-field-of-view (WFoV) multi-object spectrometer (MOS), and a complimentary WFoV camera, as well as high-resolution point source Echelle spectrometers (R~40,000). They do not require that the OTA deliver diffraction limited performance over the extent of the instrument wavelength range (115nm to 400nm). The camera and spectrometer each cover a field of view of ~ 1000 arcsec by ~ 1000 arcsec compared to ~ 150 arcsec by ~ 150 arcsec for WFC3 on the Hubble Space Telescope (HST). Thus, the AW (etendue) factor for CETUS is ~700 m^2-arcmin^2, compared to the AW factor for WFC3 on HST which is ~ 25 meter^2-arcmin^2. Thus, CETUS provides a factor of ~30 higher etendue than HST.